Colin Sullivan

Colin Sullivan (1970-2006) was a Massachusetts State Police trooper who was secretly affiliated with the Costello Mob of Boston. He was killed by a former police colleague in 2006 after the colleague discovered his corruption.

Early life
Colin Sullivan was born in South Boston, Boston, Massachusetts in 1970, the son of a janitor of Irish descent. Sullivan grew up in poverty, and, during his childhood, he met his father's friend, Irish Mob boss Francis Costello, at the Park Luncheonette. Costello had the store owner give Sullivan several free groceries, and he told Sullivan that, if he wanted to earn some extra money, he could work for him. Sullivan worked as a car mechanic for Costello, who groomed Sullivan to be one of his lieutenants. Sullivan saw Costello as a father figure to him, and he would decide to follow in his footsteps as a criminal.

State trooper
Costello later had Sullivan become his mole in the Massachusetts State Police, and Sullivan graduated from the police academy in 2006. During training, he befriended trooper James Barrigan, who, unbeknownst to Sullivan, was Costello's other mole in the police. Sullivan was accepted into the Special Investigations Unit under George Ellerby and assigned to the fight against organized crime, especially Costello's mob. Sullivan also begam a romance with police psychiatrist Madolyn Madden, who coincidentally had a relationship with undercover cop Billy Costigan, who was infiltrating Costello's mob. After Costello escaped a sting operation, each mole became aware of the other's existence, and Sullivan was asked to find the rat. At a porn theater, Costello gave Sullivan an envelope containing personal information on his crew members, and Costigan chased Sullivan through Chinatown as he left the theater, although neither mole knew the other's identity at the end of the chase. Sullivan then had police captain Oliver C. Queenan tailed to a rooftop meeting with Costigan, but Costello's thugs failed to kill Costigan, instead killing Queenan. Sean Dignam, the liaison between Ellerby's SIU and Queenan's undercover unit, resigned rather than work with Sullivan, becoming suspicious of him due to his having Queenan followed.

Using Queenan's phone, Sullivan succeeded in reaching Costigan, who refused to abort his undercover mission, despite Queenan's death. Sullivan then learned from Queenan's diary of Costello's role as an FBI informant, causing him to worry about his mole status being revealed. With Costigan's help, Sullivan tracked Costello down to a cocaine drop-off at the Sheffield Warehouse, and Costello and most of his crew were killed, with Sullivan gunning Costello down after Costello revealed his informant status. Sullivan was applauded as a hero by his colleagues, but Costigan later discovered Costello's envelope on Sullivan's desk and fled, realizing that Sullivan was the mole. Sullivan retaliated by erasing Costigan's records from the police computer system.

Later, Sullivan found Madden listening to a recording of Costello and Sullivan's illegal conversations, which had been given to her by Costigan. Madden broke up with Sullivan, who then contacted Costigan; Costigan revealed that Costello had recorded his conversations, that Costigan had been granted possession of the recordings, and that Costigan intended to implicate Sullivan. They agreed to meet at the building where Queenan died, and Costigan caught Sullivan off-guard and handcuffed him. As Costigan had secretly arranged, Trooper Anthony Brown appeared on the roof as well. Brown was shocked, and he drew his gun on Costigan, who exposed Sullivan as Costello's mole, and asked Brown why Dignam did not accompany him. Costigan then led Sullivan, his hostage, to the elevator, but, when it reached the ground floor, Trooper Barrigan shot Costigan in the head and then shot Brown. Barrigan told Sullivan that Costello had more than one mole in the police, and Sullivan shot and killed Barrigan before identifying him as the mole and posthumously giving Costigan the Medal of Merit.

Death
Despite Sullivan's seemingly plausible explanation for the recent series of events, neither Staff Sergeant Dignam nor Madden believed Sullivan's story, and Sullivan lived by himself in his apartment. One day, he returned to his apartment room with several paper bags of groceries, only to find Dignam, wearing a tracksuit, waiting in the room. Dignam shot and killed Sullivan as he entered the room, and he proceeded to run off, having eliminated the real mole.